Thyristor (SCR) Packages

Fig. 6.0.1 Typical SCR Packages

Thyristor is a general name for a number of high speed switching devices frequently used in AC power control and AC/DC switching, including triacs and SCRs (Silicon Controlled Rectifiers). The SCR is a very common type of thyristor and several examples of common SCR packages are shown in Figure 6.0.1. Many types are available that are able to switch loads from a few watts to tens of kilowatts. The circuit symbol for a SCR is shown in Figure 6.0.2. and suggests that the SCR acts basically as a SILICON RECTIFIER diode, with the usual anode and cathode connections, but with an additional CONTROL terminal, called the GATE. Hence the name Silicon Controlled Rectifier.

A trigger voltage applied to the gate whilst the anode is more positive than the cathode will switch the SCR on to allow current to flow between anode and cathode. This current will continue to flow, even if the trigger voltage is removed, until anode to cathode current falls to very nearly zero due to external influences such as the circuit being switched off, or the AC current waveform passing through zero volts as part of its cycle.

Fig. 6.0.2 Typical SCRConstruction & Circuit Symbol

The Silicon Controlled Rectifier (SCR)

SCRs, unlike normal two layer PN junction rectifiers, consist of four layers of silicon in a P-N-P-N structure, as can be seen in the cut-away view of a SCR in Fig 6.0.2. The addition of the gate connection to this structure enables the rectifier to be switched from a non-conducting 'forward blocking' state into a low resistance, 'forward conducting' state (see also Fig. 6.0.3). So a small current applied to the gate is able to switch on a very much larger current (also at a much higher voltage) applied between anode and cathode. Once the SCR is conducting, it behaves like a normal silicon rectifier; the gate current may be removed and the device will remain in a conducting state.

The SCR is made to conduct by applying the trigger pulse to the gate terminal while the main anode and cathode terminals are forward biased. When the device is reverse biased a gating pulse has no effect.To turn the SCR off, the anode to cathode current must be reduced below a certain critical "holding current" value, (near to zero).

A common application for SCRs is in the switching of high power loads. They are the switching element in many domestic light dimmers and are also used as control elements in variable or regulated power supplies.

Fig. 6.0.3 SCR Characteristics

SCR Characteristics

Fig. 6.0.3 shows a typical characteristic curve for a SCR. It can be seen that in the reverse blocking region it behaves in a similar way to a diode; all current, apart from a small leakage current is blocked until the reverse breakdown region is reached, at which point the insulation due to the depletion layers at the junctions breaks down. In most cases, reverse current flowing in the breakdown region would destroy the SCR.

When the SCR is forward biased however, unlike a normal diode, rather than current beginning to flow when just over 0.6V is applied, no current apart from a small leakage current flows. This is called the forward blocking mode, which extends to a comparatively high voltage called the 'Forward Breakover Voltage'. The SCR is normally operated at voltages considerably less than the forward break over voltage as any voltage higher than the forward break over voltage will cause the SCR to conduct in an uncontrolled manner; the SCR then suddenly exhibits a very low forward resistance, allowing a large current to flow. This current is 'latched' and will continue to flow until either the voltage across anode and cathode is reduced to zero, or the forward current is reduced to a very low value, less than the 'Holding Current' shown in Fig. 6.0.3. However the forward break over conduction may occur if the SCR is being used to control an AC (e.g. mains or line supply) voltage and a sudden voltage spike occurs, especially if it coincides with (or close to) the peak value of the AC. If the SCR is accidentally pushed into the forward break over condition, this can produce a sudden but short lived surge of maximum current, which could prove disastrous to other components in the circuit. For this reason it is common to find that SCRs have some method of spike suppression included, either within the SCR construction or as external components usually called a 'snubber circuit'.

The correct way of triggering the switch on of the SCR is to apply a current to the gate of the SCR whilst it is operating in the ´forward blocking region´, the SCR is then ´triggered´ and its forward resistance falls to a very low value. This produces a ´latching current´, which, due to the low forward resistance of the SCR in this mode, allows very large (several amperes) currents to flow in the ´forward conducting region´ with hardly any change in the forward voltage (notice that the characteristic curve, once the SCR is triggered is practically vertical). In this region current will flow, and may vary, but if forward current falls below the ´holding current´ value or the anode to cathode voltage is reduced to very near 0V, the device will return to its forward blocking region, effectively turning the rectifier off until it is triggered once more. Using the gate to trigger conduction in this way allows conduction to be controlled, allowing the SCR to be used in many AC and DC control systems.

Fig. 6.0.4 The SCR 'Two Transistor Model'

How the SCR Works

The SCR Two Transistor Model

The actual operation of the SCR can be described by referring to Fig. 6.0.4 (a) & (b), which shows simplified diagrams of the SCR structure with the P and N layers and junctions labelled. To understand the operation of a SCR, the four layers of the SCR can theoretically be thought of as a small circuit comprising two-transistors (one PNP and one NPN) as shown in Fig. 6.0.4 (b). Notice that layer P2 forms both the emitter of Tr1 and the base of Tr2, while layer N1 forms the base of Tr1 and the collector of Tr2.

The 'Off' Condition

Referring to the Fig. 6.0.4(c), with no gate signal applied and the gate(g) at the same potential as the cathode (k), any voltage (less than forward break over voltage) applied between the anode(a) and cathode(k) so that the anode is positive with respect to the cathode will not produce a current through the SCR. Tr2 (the NPN transistor) has 0v applied between base and emitter so will not be conducting, and as its collector voltage provides the base drive for Tr1 (the PNP transistor), its base/emitter junction will be reverse biased. Both transistors are therefore switched off and no current (apart from a tiny reverse leakage current) will be flowing between the SCR anode and cathode, and it is operating in its forward blocking region.

Triggering the SCR

When the SCR is operating in the forward blocking region (see the SCR characteristics in Fig. 6.0.3), if the gate and therefore the base of Tr2, see Fig 6.0.4(c) is made positive with respect to the cathode (also Tr2 emitter) by the application of a gating pulse so that a small current, typically a few µA to several mA depending on SCR type, is injected into Tr2 base, Tr2 will turn on and its collector voltage will fall. This will cause current to flow in the PNP transistor Tr1 and a rapid rise in voltage at Tr1 collector and therefore at Tr2 base. Tr2 base emitter junction will become even more forward biased, rapidly turning on Tr1. This increases the voltage applied to Tr2 base and keeps Tr2 and Tr1 conducting, even if the original gating pulse or voltage that started the switch on process is now removed. A large current will now be flowing between the P1 anode(a) and N2 cathode(k) layers.

The resistance between anode and cathode falls to near zero ohms so that the SCR current is now limited only by the resistance of any load circuit. The action described happens very quickly, as the switching on of Tr2 by Tr1 is a form of positive feedback with each transistor collector supplying large current changes to the base of the other.

As Tr1 collector is connected to Tr2 base, the action of switching on Tr1 virtually connects Tr2 base (the gate terminal) to the high positive voltage at the anode(a). This ensures that Tr2 and therefore Tr1 remain conducting, even when the gating pulse is removed. To turn the transistors off, the voltage across the anode(a) and cathode(k) must either have its polarity reversed, as would happen in an AC circuit at the time when the positive half cycle of the AC wave reached 0V before going negative for the second half of its cycle or, in a DC circuit the current flowing through the SCR is switched off. In either of these cases the current flowing through the SCR will be reduced to a very low level, below the holding current level (shown in Fig. 6.0.3), so the base emitter junctions no longer have sufficient forward voltage to maintain conduction.

Fig. 6.0.5 Low Voltage SCR Supply

Demonstrating SCR Operation

Because SCRs are normally used for controlling high power high voltage loads, this presents considerable risk of electric shock to users in any experimental or educational environments. The circuits described in the following web pages of Module 6 however, are designed to demonstrate the various control methods used with SCRs using low voltage (12VRMS) AC as illustrated in Fig. 6.0.5 rather than exposing the user to the dangers of using mains (line) voltage. Note that the circuits shown in this module are intended as low voltage demonstrations only, not as working control circuits for mains (line) circuits. For real working examples you should consult application notes produced by SCR manufacturers.

The section of the circuit containing the SCR (a C106M SCR), together with a 33R current limiting resistor and a 12V 100mA lamp is constructed on a small piece of Veroboard (protoboard), which can be easily attached to a breadboard using 'Blu Tack' or similar temporary adhesive, allowing various drive circuits to be constructed experimentally on the breadboard. The SCR is supplied with AC via a double pole switch and a 230V to 12V isolating transformer (a small medical isolation transformer is ideal) with a 250mA fuse in the secondary circuit, all housed in a double insulated box.

Fig. 6.0.6 Low Voltage SCR Supply Circuits

A bridge rectifier is contained within a separate insulated enclosure with a 1K8 wirewound resistor connected across the output to ensure there is always some load present. This ensures that output waveforms of the 12V full wave rectified output can be reliably displayed on an oscilloscope. These separate circuits, illustrated in Fig 6.0.6 are simply constructed and comprise a useful set for demonstrating and experimenting with different types of SCR or power supply operation at a low voltage.